1. Technical Field
The present disclosure relates to a substrate-level assembly (usually known as “wafer-level package”) for an integrated device and, in particular, a sensor device, as well as to a corresponding manufacturing process and the related integrated device.
2. Description of the Related Art
Semiconductor sensors are known (for example, pressure sensors, inertial sensors, microphones, or gas sensors) which are made with microfabrication techniques and whose operation is based upon the presence of a membrane that is suspended over a cavity.
For example, EP 1 577 656, filed in the name of the present applicant, describes a pressure sensor and a manufacturing process thereof. In detail (FIG. 1), the pressure sensor, designated by 1, is integrated in a substrate 2 made of semiconductor material, in particular monocrystalline silicon, having a top surface 2a. A buried cavity 3 is formed within the substrate 2, and is separated from the top surface 2a by a flexible and deformable membrane 4 suspended over the buried cavity 3 (in particular, the expression “buried cavity” denotes herein a cavity that is formed within a single body of semiconductor material at a distance from a top surface thereof). The buried cavity 3 is in this case also isolated and entirely contained within the substrate 2. Transducer elements 5, namely, piezoresistors formed by diffusion or implantation of dopant atoms, are arranged within the membrane 4, detect deformations of the membrane 4 (due to an applied pressure), and generate corresponding electrical signals as a function of the pressure to be detected. In brief, the manufacturing process of the pressure sensor 1 envisages: forming, within the substrate 2, a plurality of deep trenches, separated from one another by separation walls made of semiconductor material; then carrying out an epitaxial growth in a de-oxidizing environment so as to form an epitaxial layer, which closes the deep trenches at the top; and, finally, carrying out a thermal annealing step so as to form the buried cavity 3. A thin silicon layer remains above the buried cavity 3, and is constituted partly by epitaxially grown silicon atoms and partly by migrated silicon atoms; this silicon layer forms the membrane 4.
European patent application EP 05425028.7, filed in the name of the present applicant on Jan. 25, 2005, describes a piezoresistive accelerometer and a corresponding manufacturing process. In detail (FIG. 2), the piezoresistive accelerometer, designated by 10, has a structure substantially similar to that of the pressure sensor 1 described above, so that parts that are similar are designated by the same reference numbers, and moreover has an inertial mass 11, formed on the membrane 4, in particular approximately at the geometrical center of the membrane 4. The inertial mass 11 is constituted by welding paste, for example of silver, tin, copper, lead, gold, or other high-density metals, preferably having a density higher than 7000 kg/m3. For example, the inertial mass 11 comprises a cylindrical base portion and a hemispherical top portion, and has a radius of between 100 μm and 200 μm and a thickness of between 50 μm and 350 μm (given a side of the membrane 4 of approximately 500 μm, the ratio between the radius of the inertial mass 11 and the side of the membrane 4 is between 20% and 40%).
The inertial mass 11 is deposited through a metal mesh, made, for example, of nickel or steel, having suitable openings in positions corresponding to the areas where the welding paste is to be deposited. Furthermore, the deposition is accompanied by a temperature increase step, during which the inertial mass 11 adheres to the top surface of the membrane 4, assuming, after cooling, the described shape.
The center of gravity of the inertial mass 11 is placed outside of the membrane 4, so that, in use, an acceleration acting on the accelerometer 10 determines a momentum on the inertial mass 11, which causes inclination thereof in a corresponding direction. The displacement of the inertial mass 11 causes a deformation of the membrane 4 and a variation in the resistivity of the piezoresistive elements 5, whence an appropriate detection circuit determines the amount of the acceleration acting on the accelerometer 10.
The aforesaid patent application No. EP 05425028.7 further discloses (FIG. 3) integration of the pressure sensor 1 and of the accelerometer 10 described above in separate and distinct surface portions of a same substrate 2 of semiconductor material, in particular to obtain a pressure monitoring system 15, for example, a so-called tire-pressure monitoring system (TPMS) for monitoring the inflating pressure of a tire for a vehicle. In use, the pressure monitoring system 15 is installed on the inside surface of a tire, and the pressure sensor 1 measures the state of inflation thereof, whilst the accelerometer 10 performs a wake-up function, by supplying a start-of-measurement signal to the pressure sensor 1 and a data-collection signal to an electronic circuit coupled thereto. In particular, the accelerometer 10 detects a centrifugal acceleration of the tire during rotation. An acceleration of intensity greater than a pre-set threshold is representative of a condition of movement of the vehicle, and consequently causes the start of pressure monitoring, so limiting monitoring to time intervals during which the vehicle is moving.
Moreover, MEMS microphone sensors are known, an example of which is shown in FIG. 4. The microphone sensor, designated by 16, is again integrated in a substrate 2 made of semiconductor material, having a top surface 2a, and comprises a buried cavity 3 formed within the substrate 2 and separated from the top surface 2a by a membrane 4 suspended over the buried cavity 3. The membrane 4 is fixed and has a plurality of holes (not shown) allowing the passage of air from the external environment to the buried cavity 3. A sensor diaphragm 17, which is flexible and free to move as a result of the air pressure, separates the buried cavity 3 from a back-chamber 18 formed at the back of the substrate 2. The membrane 4 and the sensor diaphragm 17 form two facing plates of a sensing capacitor, whose capacitance varies as a function of their relative distance. In use, the sensor membrane 17 undergoes deformation as a result of sound waves reaching the buried cavity 3, thus causing a corresponding variation of the capacitance value of the sensing capacitor.
The dimensions of the sensors described are particularly small, namely, in the region of 0.8 mm×0.8 mm×0.3 mm (length×width×thickness), or in the region of 2 mm×2 mm×0.3 mm, so that traditional packaging techniques do not prove advantageous, and in particular packages of a traditional type, of a molded or pre-molded type, prove to be of excessive encumbrance and in any case not optimized for applications, such as automotive or consumer applications, which require size minimization. For example, existing packages for MEMS microphones envisage the use of a rather bulky metallic casing (or made by a combination of FR4 and a metallic material) which protects and electrostatically shields the sensor die. Moreover, packages of the traditional type are not optimized in terms of manufacturing costs.
On the other hand, the tendency to use alternative packaging techniques for integrated devices is known, said techniques enabling a reduction in the overall dimensions of the resulting electronic devices, and a simultaneous reduction in the manufacturing costs. In particular, the so-called “wafer-level packaging” technique is known, which envisages formation of a protection layer directly on top of a layer of semiconductor material housing integrated devices, to mechanically protect the integrated devices.